Lignocellulosic biomass is plant biomass that is composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are carbohydrate polymers that are tightly bound to the lignin. Lignocellulosic biomass can be grouped into four main categories: (1) agricultural residues; (2) energy crops; (3) wood residues, including sawmill and paper mill discards; and (4) municipal paper waste. Lignocellulosic biomass represents a potentially sustainable source of fuel and commodity chemicals, offers economic advantages over corn starch for the production of biofuels, and could contribute to carbon sequestration without impacting food crop prices [1]. Lignocellulosic biomass could satisfy the energy needs for transportation and electricity generation, while contributing to carbon sequestration and limiting the accumulation of greenhouse gases in the atmosphere.
Potential feedstocks of lignocellulosic biomass are abundant and include crops (e.g. corn and sugarcane), agricultural wastes, forest products (e.g. wood), grasses, and algae. Among the feedstocks, wood has been widely used for the production of paper, as a construction material, and as a solid fuel. Wood is composed mainly of cellulose, hemicellulose, and lignin. Lignin is an amorphous network of crosslinked phenylpropanoid units.
The conversion of lignocellulosic biomass into liquid fuels and/or other commodity chemicals typically includes the following steps: (1) pretreatment; (2) hydrolysis of cellulose and hemicellulose into fermentable sugars; and (3) fermentation of the sugars into the liquid fuels (e.g. ethanol) and other commodity chemicals. The pretreatment is energy-intensive, but necessary due to the complex structure of the plant cell wall and the chemical resistance of lignin, which limits the access of enzymes to cellulose. An ideal pretreatment should break the lignocellulosic complex, increase the active surface area, and decrease the cellulosic crystallinity while limiting the generation of inhibitory by-products and minimizing hazardous wastes and wastewater.
A major bottleneck in the large-scale conversion of biomass to biofuels is the pretreatment delignification process that provides enzymes access to cellulose, the main source of fermentable sugars. Most current pretreatments, such as ammonia fiber explosion, alkaline hydrolysis, and acid hydrolysis, require high temperatures that increase the operation costs and generate toxic byproducts [2, 3]. The pretreatment is also the most expensive step in the conversion of lignocellulosic biomass to ethanol. Less expensive pretreatments that are environmentally friendly are desirable.
Ionic liquids are molten salts with melting points at or below a temperature of 100° C. They are characterized by their high conductivity, high density, high thermal conductivity, high thermal stability, and extremely low vapor pressure. Alternative pretreatments involving ionic liquids have been investigated because ionic liquids can dissolve in a few hours various native biomasses that include corn stalk [4, 5], rice straw [4, 5], pine [4, 5, 6, 7, 8], oak [6, 7], spruce [8, 9, 10], maple [11], switchgrass [12], and poplar [7]. At elevated temperatures, typically above 90° C., ionic liquids can dissolve cellulose, lignin, native switchgrass, and wood sawdust ground from spruce, pine, and oak. Furthermore, ionic liquids can be recycled at high yields for further use. Most of these reported studies in ionic liquids were conducted at high temperatures on a conventional heating plate. In a typical recycling process, cellulose-rich wood extracts are precipitated and filtered out. The lignin and other extracts are removed with multiple washings and solvent evaporation. Regenerated cellulose from an ionic liquid solution of wood may have a lower degree of polymerization and crystallinity, which facilitates its hydrolysis. A few microbial celluloses remain active at an ionic liquid concentration of about 30%.
The great potential of ionic liquids is due to their low vapor pressure, thermal stability and flexibility because many anion-cation combinations are possible. A few celluloses can tolerate high concentrations of ionic liquids [13, 14, 15].
Microwave irradiation has been increasingly used in chemistry to reduce reaction times from several hours to less than a minute in some cases [16]. It was also applied to the pyrolysis of pine wood pellets [17]. Most studies were conducted in commercial microwave ovens [4, 6, 18], with a few in microwave cavities [17] at a frequency of 2.45 GHz. In contrast to conventional heating plates that rely on conduction and convection, microwave irradiation offers several advantages, including volumetric heating and quick coupling with molecules in the sample, that lead to enhanced energy efficiency [17]. It heats materials through two main mechanisms: dielectric loss in dipolar polarization and friction during ionic conduction [16].
Dry wood has a low dielectric loss factor at temperatures up to 500° C., making it a poor microwave absorber [17]. The addition of water, a strong microwave absorber at 2.45 GHz, to wood improves the conversion of microwaves into heat [17].
Microwaves have been recently used to accelerate the dissolution of wood in ionic liquids [4, 6, 18] and acids [19] with pulses as short as a few seconds. Ionic liquids are excellent microwave absorbers because they are polar and ionic in nature [16, 20]. The use of microwave pretreatment (60×3 s pulses) before conventional heating reduced the time it takes to completely dissolve pine sawdust in 1-ethyl-3-methylimidazolium acetate (EMIMAc) by a factor of about three [6], in another study, microwave irradiation increased significantly the yield of 5-hydroxymethylfurfural and furfural produced from the dissolution of pine wood in 1-butyl-3-methylimidazole chloride, while reducing the reaction time from 60 mM (conventional heating with oil bath at 100° C.) to 3 min [4]. In these studies, the biomass was completely immersed in ionic liquid and the dissolution products had to be separated from the ionic liquid, which requires additional energy and water use. Also, due to the high cost of ionic liquids, their recycling is essential for the economic viability of an up-scaled process [21].